Last week, 45 state attorneys general banded together to call on Twitter, Shopify and eBay to stop the sale of false and stolen vaccine cards. The officials said they were monitoring the activity and were concerned that unvaccinated people would misuse the cards to attend large events, potentially spreading the virus and prolonging the pandemic.

“We’re seeing a huge market for these false cards online,” said Josh Shapiro, Pennsylvania’s attorney general, whose office has investigated fraud related to the virus. “This is a dangerous practice that undermines public health.”

The C.D.C. said it was “aware of cases of fraud regarding counterfeit Covid-19 vaccine cards.” It asked people not to share images of their personal information or vaccine cards on social media.

Facebook, Twitter, eBay, Shopify and Etsy said that the sale of fake vaccine cards violated their rules and that they were removing posts that advertised the items.

The C.D.C. introduced the vaccination cards in December, describing them as the “simplest” way to keep track of Covid-19 shots. By January, sales of false vaccine cards started picking up, Mr. Khalifah said. Many people found the cards were easy to forge from samples available online. Authentic cards were also stolen by pharmacists from their workplaces and put up for sale, he said.

Many people who bought the cards were opposed to the Covid-19 vaccines, Mr. Khalifah said. In some anti-vaccine groups on Facebook, people have publicly boasted about getting the cards.

“My body my choice,” wrote one commenter in a Facebook post last month. Another person replied, “can’t wait to get mine too, lol.”

Other buyers want to use the cards to trick pharmacists into giving them a vaccine, Mr. Khalifah said. Because some of the vaccines are two-shot regimens, people can enter a false date for a first inoculation on the card, which makes it appear as if they need a second dose soon. Some pharmacies and state vaccination sites have prioritized people due for their second shots.

One Etsy seller, who declined to be identified, said she had sold dozens of fake vaccine cards for $20 each recently. She justified her actions by saying she was helping people evade a “tyrannical government.” She added that she did not plan to get inoculated.

Vaccine proponents say they have been troubled by the proliferation of forged and stolen cards. To hold those people accountable, Savannah Sparks, a pharmacist in Biloxi, Miss., began posting videos on TikTok last month naming the sellers of falsified vaccine cards.

In one video, Ms. Sparks explained how she had tracked down the name of a pharmacy technician in Illinois who had nabbed several cards for herself and her husband and then posted about it online. The pharmacy technician had not disclosed her identity, but had linked the post to her social media accounts, where she used her real name. The video has 1.2 million views.

“It made me so mad that a pharmacist was using her access and position this way,” Ms. Sparks said. The video drew the attention of the Illinois Pharmacists Association, which said it reported the video to a state board for further investigation.

Ms. Sparks said her work had drawn detractors and vaccine opponents, who have threatened her and posted her home phone number and address online. But she was undeterred.

“They should be first in line advocating for people to get vaccinated,” she said of pharmacists. “Instead, they’re trying to use their positions to spread fear and help people circumvent getting the vaccine.”

Mr. Shapiro, the Pennsylvania attorney general, said in addition to violating federal copyright laws, the sale of counterfeit and stolen cards most likely broke civil and consumer protection laws that mandate that an item can be used as advertised. The cards could also violate state laws regarding impersonation, he said.

“We want to see them stop immediately,” Mr. Shapiro said of the fraudsters. “And we want to see the companies take serious and immediate action.”